Revenue (Disaggregation of Revenues) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 987.2	$ 916.3
Electric [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	743.4	708.7
Electric [Member] | Retail - residential [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	274.7	259.4
Electric [Member] | Retail - commercial [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	181.1	174.0
Electric [Member] | Retail - industrial [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	208.7	201.9
Electric [Member] | Wholesale [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	46.5	53.8
Electric [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	32.4	19.6
Gas [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	215.8	185.6
Gas [Member] | Retail - residential [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	131.8	110.6
Gas [Member] | Retail - commercial [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	63.6	57.0
Gas [Member] | Retail - industrial [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	5.4	5.8
Gas [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	15.0	12.2
Other Utility [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	11.1	13.2
Other Utility [Member] | Steam [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	8.4	9.4
Other Utility [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	2.7	3.8
Non-utility and Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	16.9	8.8
Non-utility and Other [Member] | Transportation and other [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	16.9	8.8
IPL [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	555.1	525.8
IPL [Member] | Electric [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	419.8	405.7
IPL [Member] | Electric [Member] | Retail - residential [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	147.5	142.2
IPL [Member] | Electric [Member] | Retail - commercial [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	116.6	111.6
IPL [Member] | Electric [Member] | Retail - industrial [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	116.1	114.8
IPL [Member] | Electric [Member] | Wholesale [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	16.9	24.3
IPL [Member] | Electric [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	22.7	12.8
IPL [Member] | Gas [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	124.6	108.1
IPL [Member] | Gas [Member] | Retail - residential [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	77.9	65.5
IPL [Member] | Gas [Member] | Retail - commercial [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	34.8	31.6
IPL [Member] | Gas [Member] | Retail - industrial [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	3.2	2.7
IPL [Member] | Gas [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	8.7	8.3
IPL [Member] | Other Utility [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	10.7	12.0
IPL [Member] | Other Utility [Member] | Steam [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	8.4	9.4
IPL [Member] | Other Utility [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	2.3	2.6
WPL [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	415.2	381.7
WPL [Member] | Electric [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	323.6	303.0
WPL [Member] | Electric [Member] | Retail - residential [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	127.2	117.2
WPL [Member] | Electric [Member] | Retail - commercial [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	64.5	62.4
WPL [Member] | Electric [Member] | Retail - industrial [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	92.6	87.1
WPL [Member] | Electric [Member] | Wholesale [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	29.6	29.5
WPL [Member] | Electric [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	9.7	6.8
WPL [Member] | Gas [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	91.2	77.5
WPL [Member] | Gas [Member] | Retail - residential [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	53.9	45.1
WPL [Member] | Gas [Member] | Retail - commercial [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	28.8	25.4
WPL [Member] | Gas [Member] | Retail - industrial [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	2.2	3.1
WPL [Member] | Gas [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	6.3	3.9
WPL [Member] | Other Utility [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0.4	1.2
WPL [Member] | Other Utility [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 0.4	$ 1.2